This invention relates to a signal processor useful with switched mode deflection systems such as utilized in television receivers.
U.S. application Ser. No. 595,809 filed July 11, 1975 by Peter Eduard Haferl and entitled "SWITCHED VERTICAL DEFLECTION SYSTEM" discloses a switched mode vertical deflection system in which two switches are operated to couple horizontal rate retrace pulses to charge a capacitor which is in parallel with the vertical deflection winding. A first switch is closed for decreasing periods during successive horizontal retrace periods during a first portion of each vertical trace interval to charge the capacitor to a decreasing voltage level of one polarity. The second switch is closed for increasing periods during a second portion of each vertical trace interval to charge the capacitor with an increasing voltage of the opposite polarity. The capacitor discharges through the vertical deflection winding, causing a sawtooth scanning current to flow through the winding. The switches may be SCR's gated into conduction during the horizontal retrace periods by pulse width modulated signals obtained from a stage in which horizontal rate sawtooth signals are modulated by a vertical rate signal. As also disclosed in the referenced application, the levels of the signals coupled to the modulators may be adjusted so that the two switches overlap in conduction for varying the amount of side pincushion distortion correction provided by the loading of the horizontal deflection circuit at the vertical rate.
The horizontal retrace voltage pulses utilized for charging the capacitor are essentially sinewave shaped. Even when higher harmonic tuning of the flyback transformer is utilized the pulse still is not rectangular but has nonlinearly shaped leading and trailing edges. In the center region of the vertical trace interval, when the gating signals cause just a portion of the retrace pulse trailing edge to charge the capacitor, the rapidly decreasing trailing edge results in a lesser charge than desired because the linearly operated modulators do not compensate for the slope on the retrace pulse. The amplitude feedback in the switched vertical circuit causes the error amplifier to react, but in a linear manner, so the modulator produces a wider gating pulse compensation. The longer closing time of the switch then passes more of the trailing portion of the retrace pulse, in which the voltage increases nonlinearly because of the pulse slope, and the charge voltage across the capacitor increases beyond that level which is necessary to linearize the gain of the amplifier and the vertical rate sawtooth scanning current in the deflection winding. The result is an instability in the amplifier during the center portion of the vertical trace interval which may produce the effect of oscillations in the amplifier and a resultant undesirable nonlinearity in the scanning current. The effect of this instability may be more pronounced when the signal levels are adjusted so that the two switches overlap in conduction at the center portion of the vertical trace interval.